Tue, December 01, 2020 Turnagain Pass Avalanche Forecast

Tue, December 1st, 2020

Alpine

Above 2,500'

4 - High

Treeline

1,000'-2,500'

4 - High

Below Treeline

Below 1,000'

4 - High

0 - No Rating

1 - Low

2 - Moderate

3 - Considerable

4 - High

5 - Extreme


Travel Advice	Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.	Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.	Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential.	Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.	Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches	Natural and human-triggered avalanches unlikely.	Natural avalanches unlikely; human-triggered avalanches possible.	Natural avalanches possible; human-triggered avalanches likely.	Natural avalanches likely; human-triggered avalanches very likely.	Natural and human-triggered avalanches certain.
Avalanche Size and Distribution	Small avalanches in isolated areas or extreme terrain.	Small avalanches in specific areas; or large avalanches in isolated areas.	Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.	Large avalanches in many areas; or very large avalanches in specific areas.	Very large avalanches in many areas.

Avalanche Problem 1

Storm Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Likelihood of Avalanches

Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches

Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word	Size (D scale)	Simple Descriptor
Small	1	Unlikely to bury a person
Large	2	Can bury a person
Very Large	3	Can destroy a house
Historic	4 & 5	Can destroy part or all of a village

More info at Avalanche.org

In the past 24 hours, the mountains have received 12-18” snow, equaling 1.1-1.7” snow water equivalent (SWE), and heavy precipitation is expected through the day. As of this morning, most of this precipitation has fallen as snow, but we are anticipating rain as high as 2500 feet throughout the day. Easterly winds were sustained 15-35 mph last night, with gusts as high as 110 mph. This combination of heavy snowfall and strong winds is rapidly loading start zones, and the snowpack will quickly loose strength when liquid water (read ‘rain’) is introduced. The snowpack does not react well to rapid change, and today we are making some big changes in a hurry. We can expect to see widespread natural activity during this storm, with large avalanches running far into runout zones. Avalanches failing within the new snow will be up to 4’ deep, and that doesn’t even take into account the potential for avalanches failing on weak layers deeper in the snowpack (see problem #2). At mid- to low elevations, these avalanches will involve wet snow, and they will have the potential to reach very long runout distances. There is no question about it, the current situation is dangerous, and it will get worse before it gets better. For now, we need to stay out of avalanche terrain.

Click Here for a video checking in from Seattle Ridge yesterday.

Avalanche Problem 2

Deep Persistent Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

Likelihood of Avalanches

Size of Avalanches

Signal Word	Size (D scale)	Simple Descriptor
Small	1	Unlikely to bury a person
Large	2	Can bury a person
Very Large	3	Can destroy a house
Historic	4 & 5	Can destroy part or all of a village

More info at Avalanche.org

As if we didn’t already have enough to worry about with the storm snow issues, we also still have weak, faceted snow deep in the snowpack in many locations in our advisory area. This storm is placing a heavy burden on weak snow, and it is possible that we will see avalanches failing over 6′ deep in the snowpack. For today, we have plenty of clear signs indicating very dangerous conditions and avalanche terrain should be avoided entirely.

Tincan. We still don’t trust the weak snow near the ground. Photo: Eric Roberts. 11.30.2020

Weather

Tue, December 1st, 2020

Yesterday: In the past 24 hours, the mountains have received 12-18” snow, equaling 1.1-1.7” snow water equivalent (SWE), with lower totals at lower elevations. Easterly winds were sustained 15-35 mph last night, with gusts as high as 110 mph.

Today: As of this morning, most of this precipitation has fallen as snow, but we are anticipating rain as high as 2500 feet throughout the day. We will receive up to 12-18” more snow by the end of the day, equaling 1.0-1.2” water. Below 2000’, almost all of this will fall as rain, with .75″ rain expected in Girdwood. Winds will be 15-25 mph out of the southeast, with high temperatures in the low- to mid 30’s.

Tomorrow: This storm will taper off tonight with low temperatures hovering in the low- to mid 30’s. Lingering precipitation will be mixed rain and snow, with 1-2” snow and about 0.1” water. South winds will be 5-15 mph, and high temperatures will be in the low- to mid 30’s.

PRECIPITATION 24-hour data (6am – 6am)

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	28	17	1.7	74
Summit Lake (1400′)	18	6	0.6	27
Alyeska Mid (1700′)	30	13	1.15	73

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	19	ENE	34	110
Seattle Ridge (2400′)	24	ESE	15	43

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
10/27/24	Turnagain	Observation: Tincan	Michael Kerst
10/21/24	Turnagain	Observation: Turnagain Pass Road Observation	Trevor Clayton
10/19/24	Turnagain	Observation: Tincan – Below Todds Run	Andy Moderow
10/18/24	Turnagain	Observation: Taylor Pass	Eli Neuffer
10/15/24	Turnagain	Observation: Tincan Common	John Sykes Forecaster
10/14/24	Turnagain	Avalanche: Tincan	CNFAC Staff
05/13/24	Turnagain	Observation: Eddie’s, Sunburst, Seattle, Cornbiscuit, Pete’s South	H Thamm
05/13/24	Turnagain	Observation: Turnagain Pass non-motorized side	Amy Holman
05/12/24	Turnagain	Observation: Warm up Bowl	Tony Naciuk
05/07/24	Turnagain	Observation: Turnagain Pass Wet Slabs	A S

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